The present invention relates to an optical amplifier system in which multiple laser cavities are utilized to control the gain of an erbium doped fiber amplifier (EDFA). More specifically, the present invention is directed to an optical amplifier comprising a gain medium such as an erbium doped optical fiber which provides gain for an optical signal propagating therein. The gain medium also provides gain for a plurality of laser cavities (e.g. first and second laser cavities) which simultaneously oscillate at individual (e.g. first and second) wavelengths. The inventive optical amplifier results in reduced variations in gain spectrum as a function of input signal power, as a function of wavelength, and as a function of time. By varying the optical attenuation in one or more of the individual laser cavities it is possible to vary the gain spectrum of the gain medium at the corresponding individual wavelength and thus control the shape of the gain spectrum of the gain medium.
Dense wavelength division multiplexed (DWDM) optical networks have been widely accepted as the choice for next generation, high capacity transmission systems. The successful operation of such systems will require optical amplifiers such as erbium doped fiber amplifiers (EDFAs) that can provide uniform and stable gain for optical signals. Such demands have led to various techniques to engineer amplifiers with these characteristics. Because the intrinsic gain profile of an EDFA is not uniform, two approaches have been used to obtain a flat gain spectrum over the signal band. The first approach modifies the gain medium to reduce the intrinsic gain ripple. An example of this is the development of the erbium doped fluoride fiber amplifier (EDFFA), which provides less gain ripple than an erbium doped silica fiber amplifier (EDSFA). The second approach incorporates external devices to correct gain profiles. These devices can be either active or passive ones. Current devices for gain spectrum shaping are mostly passive filters that tailor the gain spectrim into a flat top.
In addition, future wavelength division multiplexed optical networks will require erbium doped fiber amplifiers that can provide constant gain regardless of the total input signal power, e.g. number of channels present. Recently, several techniques have been shown to provide relatively good automatic gain control in optical amplifiers. Typically, the techniques can be classified into two distinct groups: the first, electrical automatic gain control (EAGC) and the second, optical automatic gain control (OAGC). In OAGC, a single laser cavity is formed in either a ring or standing wave configuration. In spite of its success, signal band inhomogeneity results in relatively large gain variations (as much as 1 dB) when signal power is changed, e.g. in the presence of channel add/drops. Such unwanted gain changes could impose serious system penalties in communication networks.
In addition, in wavelength division multiplexed networks, temporal (or relaxation) oscillations in the gain spectrum result when one of the wavelengths is dropped (or added). These oscillations in the gain spectrum in turn result in relaxation oscillations in the amplitudes of the surviving wavelengths. Such relaxation oscillations are undesirable in optical communication systems.
In view of the foregoing, it is an object of the invention to provide an optical amplifier in which variations in the gain spectrum in response to changes in signal input power are reduced. More particularly, it is an object of the invention to provide an optical amplifier in which the gain is relatively constant, regardless of changes in total input signal power due to the adding or dropping of channels.
It is also an object of the invention to provide an optical amplifier system in which variations in gain spectrum as a function of wavelength and as a function of time are reduced so as to increase uniformity and stability of gain across the optical signal band.
It is a further object of the invention to provide an optical amplifier in which it is possible to control the shape of the gain spectrum in the optical signal band.
In accordance with an illustrative embodiment of the invention, an optical amplifier comprises a gain medium, such as an erbium doped optical fiber, for providing optical gain to an optical signal propagating therein. A pump laser is coupled to the gain medium.
The gain medium provides gain for first and second laser cavities coupled thereto. The cavities may be ring cavities or linear cavities. The following description pertains to a ring cavity. The first cavity includes a first filter transmissive at a first optical wavelength and a first optical attenuator which may be variable. The second cavity includes a second filter transmissive at a second optical wavelength and a second optical attenuator which may be variable. The first laser cavity oscillates at the first optical wavelength and the second laser cavity simultaneously oscillates at the second optical wavelength. Both the first and second wavelengths are in the signal band of the gain medium. In this case, the first optical attenuator compensates for the gain of the gain medium at the first optical wavelength and the second optical attenuator simultaneously compensates for the gain of the gain medium at the second optical wavelength. The first and second simultaneously oscillating laser cavities reduce variations in the gain of the gain medium both as a function of input signal power, e.g., when channels are added or dropped, as a function of wavelength (i.e. across the signal wavelength band), and as a function of time, (i.e. relaxation oscillations in surviving wavelengths are reduced when one wavelength is dropped in a multiple wavelength system).
In an alternative embodiment, additional laser cavities; which oscillate at additional wavelengths in the signal band, are coupled to the gain medium.
The inventive optical amplifier may be operated so as to control the shape of the gain spectrum of the gain medium across the signal wavelength band. The method comprises the step of varying the variable optical attenuator in one or more of the laser cavities coupled to the gain medium. This in turn causes a variation in the gain of the gain medium at the corresponding wavelength and nearby wavelengths via spectral inhomogeneity.